Vulcanization molding of a pneumatic tire is performed such that: a raw cover is set in a vulcanization mold; then, a bladder arranged on the inner side of the raw cover is expanded so that the outer surface of the raw cover is pressed against the inner face of the vulcanization mold; followed by heating and pressurizing the raw cover at predetermined temperature and pressure for a predetermined time.
In this vulcanization molding, when air remains in the inner face or the surface of the raw cover, this causes degradation in the external appearance quality serving as a product and in the durability of the pneumatic tire. Thus, for example, a method that a vent hole is provided in the vulcanization mold and then air vent is performed (e.g., Patent Document 1) and a method that a vent piece is embedded in a prepared hole formed in the mold and then air vent is performed have widely been employed.
Nevertheless, in these methods, occurrence of spew caused by rubber extruded into the vent hole or the hole of the vent piece cannot be avoided. Among these, the spew caused by the vent hole need be removed in the manufacturing process. Further, as for the spew caused by the vent piece, when the number of occurrences increases, unpreferable external appearance is caused.
As methods for suppressing such occurrence of spew, the method of dividing the mold into a plurality (e.g., Patent Document 2) and the method of employing a spring-shaped vent piece (a spring vent) have been proposed.